Multicarrier modulation techniques such as orthogonal frequency division multiplexing (OFDM) are being used for wired and wireless communication (e.g. WiMAX or LTE) in local area networks or cellular networks. In general, multicarrier modulated symbols may be divided into subcarriers (also known as tones) that occupy a smaller frequency band in the frequency spectrum. It is possible to transmit and receive data to and from one or more sources (e.g., users) via the subcarriers.
The peak-to-average power ratio (PAPR) refers to the square of the peak value divided by the square of the RMS value. OFDM transmission is a technique capable of efficiently transmitting a large amount of data using many orthogonal subcarriers. The PAPR for the OFDM symbol can be expressed as Equation 1:
                    PAPR        =                                            max                              n                =                1                            N                        ⁢                          (                                                                                      z                    n                                                                    2                            )                                                          1              N                        ⁢                                          ∑                                  n                  =                  1                                N                            ⁢                                                                                      z                    n                                                                    2                                                                        Equation        ⁢                                  ⁢        1            
In Equation 1, zn is the nth sample of an OFDM symbol z, and the max function of the numerator determines the maximum value of |zn|2 for n=1, . . . , N.
The OFDM transmission scheme is vulnerable to the PAPR because it uses multiple carriers. For example, if an OFDM symbol has a large PAPR, distortion may occur in the power amplification stage of the amplifier. One or more relatively large samples of an OFDM symbol may require the amplifier to be driven above the maximum output level. In addition, the input-output relationship of the amplifier may be nonlinear, causing nonlinear distortion of the OFDM symbol. When the maximum output level is reached, the amplifier typically clips the input sample and places an upper limit so that the maximum output level is not exceeded, which can cause another nonlinear distortion of the output signal. Nonlinear distortion affects the quality of the signal and, as a result, the receiver may have difficulty in recovering the transmitted data.
A number of schemes may be used to reduce the effects of nonlinear distortion by the amplifier or to eliminate nonlinear distortion.
First, the transmitter can use an amplifier capable of outputting higher power levels. That is, the amplifier with a large output range may operate with a significant backoff (upper margin) to ensure that it remains in the linear operating range even when a peak signal occurs. However, using an amplifier having a large output range is inefficient in terms of cost.
As another scheme, the transmitter may be configured to perform amplification in multiple stages to achieve the desired output level. In this scheme, each stage includes an amplification step and a filtering step. In the amplification step, relatively small clipping is performed on peak samples of each OFDM symbol. In the filtering step, each OFDM symbol is filtered to reduce distortion and sidelobes caused by clipping. This clipping and filtering process may be repeated until the desired amplification level is reached. By amplifying the signal in a multistage manner, the amount of distortion can be reduced in comparison to an equivalent single-stage amplifier.
As another scheme, multiple pseudorandom scrambling sequences can be applied to the OFDM signal in the frequency domain (e.g., output subcarrier mapper). For example, the transmitter may select a scrambling sequence causing the lowest PAPR after IFFT processing. As the selected scrambling sequence is not known to the receiver, the scrambling sequence may be transmitted to the receiver via a different channel or may be detected by the receiver in a blind manner. In the blind manner, the receiver may compare all possible sequences to select the most probable sequence, increasing the complexity of the reception process.
As a peak cancellation scheme, tone reservation (TR) reduces the PAPR for each OFDM symbol by appropriately adjusting the cancellation signal (e.g. impulse signal) generated using reserved tones.